Current feeding arrangement for electrical apparatus having low temperature cooled conductors

ABSTRACT

This invention is concerned with a current feeding arrangement for a superconductor having a cooled standard conductor connected to it. The invention is comprised of subdividing the gas stream of the cooling medium into individual streams which each flow through a flow channel having small wall spacing. The current feeding arrangement has the advantage that at the walls of the individual flow channels a substantially lower density of charge carriers is obtained, and thus a corresponding increase in the dielectric strength of the gaseous cooling medium.

United States Patent [191 Schmidt et al.

[ 1 Nov. 19, 1974 Matthiius, Spardorf; Peter Massek, Forchheim, all ofGermany Assignee: Siemens A ktiengeseiischait,

Munich, Germany Filed: Dec. 4, 1973 Appl. No.: 421,693

Related US. Application Data Continuation of Ser. No. 314,565, Dec. l3,I972, abandonedv Foreign Application Priority Data Dec. 20, 197i Germany2163270 US. Cl. 174/21 R, 174/15 R, 174/15 C,

l74/D1G. 6 Int. Cl I-Ilv 11/00 Field of Search. l74/l R, 15 C, 16 R, 1581-1,

[56] References Cited UNITED STATES PATENTS 3,371,145 2/1968 Camillel74/l5 R 3,522,361 7/1970 Kafl a 3,688,838 9/1972 Sturm et al. l65/l05Primary ExaminerA. T. Grimley Attorney, Agent, or Firm-Kenyon & KenyonReilly Carr & Chapin [5 7 ABSTRACT This invention is concerned with acurrent feeding arrangement for a superconductor having a cooledstandard conductor connected to it. The invention is comprised ofsubdividing the gas stream of the cooling medium into individual streamswhich each flow through a flow channel having small wall spacing. Thecurrent feeding arrangement has the advantage that at the walls of theindividual flow channels a substantially lower density of chargecarriers is obtained, and thus a corresponding increase in thedielectric strength of the gaseous cooling medium.

10 Claims, 1 Drawing Figure STANDARD CONDUCTOR --TUBULAR cououn 19 22REFRIGERATOR l T 3 PUMP also M E 5 COOLING MEDIUM SUPER CONDUCTORPATENTEL NOV 1 9|974 STANDARD CONDUCTOR ---TUBULAR CONDUIT COOLINGMEDIUM SUPER CONDUCTOR CURRENT FEEDING ARRANGEMENT FOR ELECTRICALAPPARATUS HAVING LOW TEMPERATURE COOLED CONDUCTORS This is acontinuation, of application Ser. No. 314,565 filed Dec. 13, 1972, nowabandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention isconcerned with a current feeding arrangement for electricl apparatushaving conductors cooled to a low temperature, more particularly it isconcerned with a current feeding arrangement for superconductors, theend of the superconductors being connected to a standard conductor,cooled by a gaseous cooling meium.

2. Description of the Prior Art In electrical apparatus havingsuperconductors, for example, in cables, coils or machines, electriccurrent must frequently be fed from a point at a higher temperature,particularly room temperature, to the superconductor, which is cooled toa temperature below its transition temperature. Because thesuperconductor loses its superconductivity at a temperature far belowroom temperature, an electrically normal conductive metal, like aluminumor copper, is used to bridge the temperature difference, and isconnected with the superconductor at a point maintained at a temperaturebelow the transition temperature of the superconductors. The standardconductor is therefore gradually cooled, in stages, up to the connectionpoint.

The end of the superconductor, held below its transition temperature,can, for example, be arranged in a bath of a cryogenic medium, such as,a helium bath. The standard conductor is then comprised of laminationsor grids at the connection point. Such a design is shown in Review ofScientific lnstruments,"vol. 38, No. 12 (Dec. 1967), p. 1776 to 1779.Due to the thermal losses at the junction, the liquid helium evaporatesand the helium gas rises through the conductor laminations, wires or theconductor grid and removes the joulean heat and the heat flowing in fromthe outside. In the process, the helium gas is warmed up toapproximately room temperature. At the upper junction of the standardconductor with the current supply, the helium gas is collected andreturned to a refrigeration machine for liquification. Because the heatcontent of the gaseous cooling medium is well utilized in such currentfeeds, they require only a relatively small cooling effort.

If, however, high transmission rates are to be obtained, at whichsuperconducting cables become economical, one is compelled to usecorrespondingly high voltages of 220 kV and higher. This type of currentfeed utilizes contact at one end with the highly cooled conductor, whilethe other end to which the conventional current supply is connected, isat a higher temperature, preferably room temperature. The cooling mediumtherefore flows along the indvidual conductors of the current feed andis in close contact with the high voltage conductor. The refrigerationmachine for cooling medium, however, is generally at ground potential.To conduct the gas of the cooling medium away from the high-voltagepotential by an insulating tube is not directly possible because heliumat about 300K has a very low breakdown voltage.

It is an object of the invention therefore to design an exhaust,gas-cooled, current feeding arrangement in such a way that it can beoperated economically at high voltage and with a transfer of the coolinggas to the refrigeration equipment.

In the known arrangement an electric field is formed, in the exhaust gasline, between the upper junction of the standard conductor with thecurrent supply and the refrigeration equipment. This field forms freecharge carriers, the number of which is essentially determined by thepotential gradient and the length of the gas line. A highvoltage-resistant current feed therefore requires a correspondingly longgas line. In a long gas line, however, a large number of charge carriersare generated because of the well-known avalanche effect. This in turncauses a corresponding reduction in the dielectric strength of the gas.It is therefore not possible to bridge a large potential difference by acorresponding lengthening of the gas line alone.

SUMMARY OF THE INVENTION This invention is based on the discovery thatthe dielectric strength of the cooling gas can be increasedsubstantially if the number of the charge carriers in the gas arereduced by an appropriate design of the gas circulation between thestandard conductor and the refrigeration equipment. According to theinvention, this problem is solved by dividing the gas stream of theevaporated cooling medium into individual streams, each of which passesthrough a flow channel bounded by at least two walls of electricalinsulating material, whose displacment is not more than 30mm, with thepreferred range of less than 3 mm and up to 10 mm.

In a tube the density of charge-carriers formed decreases steeply in aradial direction outward from the center toward the wall, with a maximumamount occuring at the center. The effect on the gas of using electricalinsulation material in the wall is to destroy the charge carriers. Thiswall effect is used in this invention to increase the dielectricstrength of the gaseous medium. According to the invention, the gasstream therefore is subdivided into many individual flow channels, whichcan preferably be made like capillaries, the diameter of whichtheoretically should not substantially exceed the mean free path of thecharge carriers, which for gaseous helium, is about 10' cm. This walleffect, however, is also reached with a larger wall spacing, notsubstantially exceeding 30mm, especially if means are provided togenerate turbulent flow in the flow channels and thereby cause a largenumber of the charge carriers in the gas to strike the wall as the gasflows through the channel and thereby give off their charge. The partialgas stream entering the flow channel can be given a rotary flow motion.Without such means, a substantial reduction in the number of chargecarriers is obtained if the gas flows through tubular channels having adiameter substantially equal to 0.3 mm. Such indivudual channels for thepartial streams can be produced by using a woven grid in the tubularconduit for the gas, or by using a fiber-like insert in the form of awick in the conduits. The gas then flows along the individual fibers andthe wall effect is obtained. Alternatively the capillary flow channelscan also be formed from pores in an electrical insulating, gas-permeablematerial. Another alternative is to fill the flow channels with apowdered, electrical insulating material, the grain size of which ischosen to obtain the required,

capillary-like flow channels. A still further alternative is to designthe flow channels from molecular sieves.

BRIEF DESCRIPTION OF THE DRAWING To explain the invention in furtherdetail, reference is made to the drawing, which illustratesschematically an emobodiment of a current feed arrangement construcedaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the FIGURE, the end of thesuperconductor 2 is connnected to a standard conductor by member 4 and acooling medium bath 6, containing helium in container 8 which alsoencloses a laminated end 10 of a standard conductor 12, the terminal ofwhich is 14. In accordance with the invention, the gas flow isdistributed over individual gas lines 18 between two pipe lines 16 and20. Connected to pipe line 20 is a refrigeration machine which isconnected by pipe line 24 to container 26, from which the liquid coolingmedium 28 can be fed to the bath 6 by a feed pipe 30, a pump 32 and feedline 34.

The junction 4 between the laminations 10 of the standard high-voltageconductor 12 and the superconductor 2 is located in the boiling heliumof bath 6. Because of the joulean heat of the current-carryinglaminations l and the heat inflow from the standard conductor 12 throughthe laminations, part of the helium evaporates. The gas coolslaminations by rising upward between them into the upper part of thecontainer 8 and acts as a gas cushion. From there it is fed through thepipe line 16 to the tubular conduits 18, formed of electrical insulatingmaterial which are preferably subdivided for the gas flow intoindividual capillaries formed from a powdered filling, particularlyglass powder or glass wool. On the way through these capillaries, manyof the charge carriers generated by the electric field are anihilated bythe wall effect thereby increasing the dielectric strength of thecooling gas to such and extent that voltage breakdown in the gaseouscooling medium is prevented. The cooling medium can therefore be fed bypipeline 20 to refrigerator 22, where it is reliquified and fed throughpipeline 24 to the supply tank 26.

Another particularly advantageous embodiment of the current feedingarrangement of the present invention consists in providing filters 17and 19 at the upper and the lower terminals of the fillings for thepipes 18. These filters can be made from glass frit, the openings ofwhich are preferably smaller than the grain size of the insulatingpowder in the pipes 18.

Material to fabricate the capillaries in the tube condiuts 18 can beinsulating powder of quartz, ceramic material or plastic. Well suited isglass powder having a grain diameter of approximately 50 to 150 pm.

By using a diameter in tubes 18 smaller than 10 mm, and moreparticularly, smaller than 3 mm, with additional special devices, onecan obtain a good wall effect within the gas stream. For larger tubediameters or a larger distance between the walls of fiat tubularcondiuts, the wall effect can advantageously be increased by eitherintroducing a turbulent flow gas stream into the individual tubes 18 orproviding additional means inside the tubes 18. Such means can be ahelix, which during the flow of the gas in the tube, permits as large apart as possible of the gas coming into contact with the wall.

In the example of the embodiment a current feeding arrangement isdescribed cooled by liquid or gaseous helium, but it should be realizedthat other boiling gases are also suitable. For a superconductor,hydrogen can also be considered, and for conductors cooled down to lowtemperatures, nitrogen or neon can also be used.

In the foregoing, the invention has been described in reference tospecific exemplary embodiments. It will be evident, however, thatvariations and modifications, as well as the substitution of equivalentconstructions and arrangement for those shown for illustration, may bemade without departing from the broader scope and spirit of theinvention as set forth in the appended claims. The specification anddrawing are accordingly to be regarded in an illustrative rather than ina restrictive sense.

What is claimed is:

1. A current feeding arrangement for electrical apparatus havingconductors, cooled to a low temperature, connected to a standardconductor by a connecting means, in which the connecting means iscontained in a container and is cooled by a gasified liquid coolant, v

the improvement comprising connecting the end of the container locatedat the end of the connecting means disposed away from the conductorcooled to a low temperature, to a plurality of flow channels each havingat least two walls covered by an electrical insulating material, andseparated by a displacement of not more than 30 mm.

2. A current feeding arrangement for electrical apparatus as in claim 1in which the displacement of said walls is not substantially greaterthan 3 mm.

3. A current feeding arrangement for electrical apparatus as in claim 1further comprising means for generating a rotary flow of the gas throughthe flow channels to increase the frequency of contact between chargecarriers and the insulating material.

4. A current feeding arrangement for electrical apparatus as in claim 1in which the displacement of said walls in not substantially greaterthan 0.3 mm.

5. A current feeding arrangement for electrical apparatus as in claim 4in which the flow channels are formed from a woven grid.

6. A current feeding arrangement for electrical apparatus as in claim 4in which the flow channels are formed from the fibers of a wick ofelectrical insulating material.

7. A current feeding arrangement for electrical apparatus as in claim 4in which the flow channels are formed from the pores of an electricalinsulating, gas permeable material.

8. A current feeding arrangement for electrical apparatus as in claim 4in which the flow channels are formed from grains of an electricalinsulating powder.

which said coolant comprises helium.

UNITED STATES FATE OFFICE CERTIFICATE OF c-RREcTmN Patent No- 3,849 ,589Dated November 19, 1974.

lnv n fl Exit; Schmidt, Qt'inther Matthius, Peter Massek It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In column 1, line 11, change "electricl" to -electrical-- In column 1,line 16, change "meium" to--medium- In column 2, line 32, change"displacment" to displacement- In column gz line 58, change "indivudual"to -individual- In column 3, line 7, change "emobodiment" to -embodimentIn colunm 3, line 8, change "construced" to constructed- In column3,line 40, change "and" to -an- In column 3, lines 53 and 61, change"condiuts" to --conduits- In column 4, line 43, change ".in" to --is-Signed and sealed this 11th day of March 1975 (SEAL) Attest:

C MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks UNITED STATES PATENT @EFHCE C RTFIQATE on con Patent No-3, 849 589 Dated November 19 1.974

Inv h fl Fritz Schmidt, Gunther B hsrmflfius,Z Peter Massek It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In column 1, line 11, change "electricl" to -electrical- In column 1,line 16, change "meium" to--medium- In column 2, line 32, change"displacment" to -displacement In colunuii i, line 58, change"indivudual" to --individual-- I In column 3, line 7, change"emobodiment" to --embodiment-- In column 3, line 8, change "construced"to constructed-- In column 3, line 40, change "and" to --an-- In colunm3, lines 53 and 61, change "condiuts" to --conduits In column 4, line43, change "in" to -is-- Signed and sealed this 11th day of March 1975.

(SEAL) Attest:

' C. MARSHALL DANN RUTH C. MASON Commissioner of Patents AttestingOfficer and Trademarks

1. A current feeding arrangement for electrical apparatus havingconductors, cooled to a low temperature, connected to a standardconductor by a connecting means, in which the connecting means iscontained in a container and is cooled by a gasified liquid coolant, theimprovement comprising connecting the end of the container located atthe end of the connecting means disposed away from the conductor cooledto a low temperature, to a plurality of flow channels each having atleast two walls covered by an electrical insulating material, andseparated by a displacement of not more than 30 mm.
 2. A current feedingarrangement for electrical apparatus as in claim 1 in which thedisplacement of said walls is not substantially greater than 3 mm.
 3. Acurrent feeding arrangement for electrical apparatus as in claim 1further comprising means for generating a rotary flow of the gas throughthe flow channels to increase the frequency of contact between chargecarriers and the insulating material.
 4. A current feeding arrangementfor electrical apparatus as in claim 1 in which the displacement of saidwalls in not substantially greater than 0.3 mm.
 5. A current feedingarrangement for electrical apparatus as in claim 4 in which the flowchannels are formed from a woven grid.
 6. A current feeding arrangementfor electrical apparatus as in claim 4 in which the flow channels areformed from the fibers of a wick of electrical insulating material.
 7. Acurrent feeding arrangement for electrical apparatus as in claim 4 inwhich the flow channels are formed from the pores of an electricalinsulating, gas permeable material.
 8. A current feeding arrangement forelectrical apparatus as in claim 4 in which the flow channels are formedfrom grains of an electrical insulating powder.
 9. A current feedingarrangement for electrical apparatus as in claim 4 in which the flowchannels are formed from a molecular sieve.
 10. A current feedingarrangement for electrical apparatus as in claim 1 in which saidconductor cooled to a low temperature comprises a superconductor, and inwhich said coolant comprises helium.